Atherosclerosis and its clinical consequences, coronary heart disease (CHD), stroke and peripheral vascular disease, represent a truly enormous burden to the health care systems of the industrialized world. In the United States alone, approximately 13 million patients have been diagnosed with CHD, and greater than one half million deaths are attributed to CHD each year. Further, this toll is expected to grow over the next quarter century as an epidemic in obesity and diabetes continues to grow.
It has long been recognized that in mammals, variations in circulating lipoprotein profiles correlate with the risk of atherosclerosis and CHD. The clinical success of HMG-CoA Reductase inhibitors, especially the statins, in reducing coronary events is based on the reduction of circulating Low Density Lipoprotein cholesterol (LDL-C), levels of which correlate directly with an increased risk for atherosclerosis. More recently, epidemiologic studies have demonstrated an inverse relationship between High Density Lipoprotein cholesterol (HDL-C) levels and atherosclerosis, leading to the conclusion that low serum HDL-C levels are associated with an increased risk for CHD.
Metabolic control of lipoprotein levels is a complex and dynamic process involving many factors. One important metabolic control in man is the cholesteryl ester transfer protein (CETP), a plasma glycoprotein that catalyzes the movement of cholesteryl esters from HDL to the apoB containing lipoproteins, especially VLDL (see Hesler, C. B., et. al. (1987) Purification and characterization of human plasma cholesteryl ester transfer protein. J. Biol. Chem. 262(5), 2275-2282)). Under physiological conditions, the net reaction is a heteroexchange in which CETP carries triglyceride to HDL from the apoB lipoproteins and transports cholesterol ester from HDL to the apoBliprotein.
In humans, CETP plays a role in reverse cholesterol transport, the process whereby cholesterol is returned to the liver from peripheral tissues. Intriguingly, many animals do not possess CETP, including animals that have high HDL levels and are known to be resistant to coronary heart disease, such as rodents (see Guyard-Dangremont, V., et. al., (1998) Phospholipid and cholesteryl ester transfer activities in plasma from 14vertebrate species. Relation to atherogenesis susceptibility, Comp. Biochem. Physiol. B Biochem. Mol. Biol. 120(3), 517-525). Numerous epidemiologic studies correlating the effects of natural variation in CETP activity with respect to coronary heart disease risk have been performed, including studies on a small number of known human null mutations (see Hirano, K.-I., Yamashita, S. and Matsuzawa, Y. (2000) Pros and cons of inhibiting cholesteryl ester transfer protein, Curr. Opin. Lipidol. 11(6), 589-596). These studies have clearly demonstrated an inverse correlation between plasma HDL-C concentration and CETP activity (see Inazu, A., et. al. (2000) Cholesteryl ester transfer protein and atherosclerosis, Curr. Opin. Lipidol. 11(4), 389-396), leading to the hypothesis that pharmacologic inhibition of CETP lipid transfer activity may be beneficial to humans by increasing levels of HDL-C while lowering those of LDL.
Despite the significant therapeutic advance that statins such as simvastatin (ZOCOR®) represent, statins only achieve a risk reduction of approximately one-third in the treatment and prevention of atherosclerosis and ensuing atherosclerotic disease events. Currently, few pharmacologic therapies are available that favorably raise circulating levels of HDL-C. Certain statins and some fibrates offer modest HDL-C gains. Niacin, which provides the most effective therapy for raising HDL-C that has been clinically documented, suffers from patient compliance issues, due in part to side effects such as flushing. An agent that safely and effectively raises HDL cholesterol levels can answer a significant, but as yet unmet medical need by offering a means of pharmacologic therapy that can significantly improve circulating lipid profiles through a mechanism that is complementary to existing therapies.
New classes of chemical compounds that inhibit CETP are being investigated at several pharmaceutical companies or are in clinical trials. No CETP inhibitors are currently being marketed. Phase III clinical trials of torcetrapib were terminated due to an increase in mortality in patients using torcetrapib in a long-term outcomes trial. The mechanism that caused the increase in mortality is not currently known. New compounds are needed to ensure that CETP inhibitors are found that have the best profile of properties that relate to safety and effectiveness. The novel compounds described herein are very potent CETP inhibitors.